Image data coding apparatus and image data server

ABSTRACT

An image data coding apparatus having a coding unit that generates coded data after generating a transform coefficient by implementing Wavelet transform on image data, includes: a storage unit that stores coded data which the coding unit generates by coding image data which have not been sub-sampled; an information obtaining unit that obtains compression-related information indicating a compression rate or a restored image quality; and a re-coding unit that changes a sub-sampling ratio of individual color components constituting the coded data stored in the storage unit in conformance to the compression-related information and generates new coded data.

INCORPORATION BY REFERENCE

The disclosure of the following priority application is hereinincorporated by reference: Japanese Patent Application No. 2001-143676filed May 14, 2001

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an image data coding apparatus thatcodes image data through Wavelet transform and an image data server thattransmits image data coded through Wavelet transform to a client via anetwork.

2. Description of the Related Art

There are image data servers in the related art that transmitpre-recorded images at a client's request.

Such image data servers include those that record in advanceuncompressed image data which have not been coded and those that recordin advance coded data obtained by compression-coding image data. Theprocedure of the processing implemented after a transmission request isreceived varies among these image data servers depending upon the formatof the pre-recorded data.

For instance, when a transmission request from a client is received, animage data server in which uncoded, uncompressed image data have beenrecorded also obtains compression-related information indicating eitherthe compression rate for the image data or the required quality of therestored image. Then, the image data server performs compression codingon the image data in conformance to the compression-related informationthus obtained and transmits the coded image data to a client.

Image data servers in which coded data have been recorded include imagedata servers in which coded data having been coded through compressionexecuted at a compression rate (approximately 1/10˜ 1/20) that allowsthe restored image to achieve a medium image quality are pre-recorded(hereafter referred to as “medium quality compression image dataservers”). Such a “medium quality compression image data server”transmits the recorded coded data as they are upon receiving atransmission request from a client.

In addition, image data servers in which coded data have been recordedmay be image data servers in which coded data obtained by compressingimage data at a compression rate (approximately ¼˜⅛) that allows therestored image to achieve a high image quality are recorded in advance(hereafter referred to as “high quality compression image dataservers”). A “high quality compression image data server” can respond toa request for transmission of a high quality image or a high resolutionimage issued by a client simply by transmitting the recorded coded dataas they are. Furthermore, the high quality compression image data servercan respond to a request for a transmission of a low quality image or alow resolution image the intent of which is to reduce the data volume byfirst decompressing the recorded coded data and thenre-compression-coding the data at the compression rate conforming to theclient's request.

As a new image data coding technology to replace the discrete cosinetransform coding technology, Wavelet transform coding has beenattracting a great deal of interest. At present, the Committee for StillImage Coding Standard jointly organized by ISO and ITU-T, is in theprocess of standardizing JPEG 2000 that adopts image data coding throughWavelet transform. It is to be noted that the standardization processfor JPEG 2000 has been carried out in six separate parts, and the part(Part 1) that defines the core technology of JPEG 2000 wasinternationally standardized in December 2000.

FIGS. 7 and 8 present a flowchart of the operation performed in an imagedata server in the related art that adopts the coding technology definedin the JPEG 2000 Part 1. It is to be noted that the coding technologydefined in JPEG 2000 Part 1 is adopted in the operation in the flowchartpresented in FIGS. 7 and 8, which is performed by an image data serverthat records in advance uncoded, uncompressed image data in the RGBcolorimetric system.

FIG. 9 is a functional block diagram of such an image data server in therelated art.

As shown in FIG. 9, an image data server 100 in the related art includesan image recording device 101 that records in advance image data, acoding unit 102 that codes the image data through compression coding anda network interface unit 103 provided to achieve an interface with aclient via a network.

The coding technology defined in the JPEG 2000 Part 1 that is adopted inthe image data server 100 in the related art is explained below inreference to FIGS. 7 through 9. It is to be noted that the followingexplanation is given by assuming that the desired level of compressionrate (level 0˜level 10) is provided to the network interface unit 103 ascompression-related information for purposes of simplification.

First, the image data server 100 repeatedly makes a decision via thenetwork interface unit 103 as to whether or not an image transmissionrequest has been issued by a client (FIG. 7 S101).

Then, upon receiving an image transmission request, the image dataserver 100 obtains the compression-related information (the level of thedesired compression rate) provided by a client via the network interfaceunit 103 (FIG. 7 S102).

The compression-related information thus obtained is provided to thecoding unit 102.

The coding unit 102 reads out from the image recording device 101 theimage data corresponding to the image to be transmitted and divides theimage data into a plurality of rectangular areas (hereafter referred toas “tile images”) (FIG. 7 S103).

Next, the coding unit 102 implements color coordinate conversion on theindividual tile images and thus generates image data in the YCbCrcolorimetric system (FIG. 7 S104).

Then, the coding unit 102 sets a sub-sampling (culling) ratio for theindividual color components (the Y component, the Cb component and theCr component) of the image data based upon the compression-relatedinformation (FIG. 7 S105), and performs sub-sampling of the image databy culling the color difference components (the Cb component and the Crcomponent) in the image data based upon the sub-sampling ratio setting(FIG. 7 S106).

For instance, if the compression-related information (the desired levelof the compression) indicates level 0˜level 3, the coding unit 102 setsthe sub-sampling ratio for Y, Cb and Cr to “4:4:4”, whereas if thecompression-related information indicates level 4˜level 7, the codingunit 102 sets the sub-sampling ratio for Y, Cb and Cr to “4:2:2”. If thecompression-related information indicates level 8 or higher, thesub-sampling ratio for Y, Cb and Cr is set to “4:2:0”. (B) of FIG. 10shows a state achieved by sub-sampling the color difference componentswith the sub-sampling ratio for Y, Cb and Cr set to “4:2:0”.

FIG. 11 illustrates the various sub-sampling ratios.

Next, the coding unit 102 breaks down each of the color components (theY component, the Cb component and the Cr component) of the sub-sampledimage data into a plurality of sub-bands (LL, LH, HL and HH)corresponding to varying resolutions, through discrete Wavelet transformimplemented along two directions, i.e., the vertical direction and thehorizontal direction (FIG. 7 S107).

The LL sub-band among these sub-bands undergoes a further discreteWavelet transform and thus is broken down into a plurality of sub-bands(LL, LH, HL and HH). Each component in the image data is broken intosub-bands, as illustrated in (C) of FIG. 10 through the discrete Wavelettransform which is repeated recursively as described above. It is to benoted that in (C) of FIG. 10, breakdown level 0 is designated to the LLsub band and each of three sub-bands, LH, HL and HH, is designated asbreakdown level 1, 2, . . . in order of the number of times ofundergoing discrete Wavelet transform, with breakdown level 1 designatedto a sub-band having undergone discrete Wavelet transform the greatestnumber of times.

Next, the coding unit 102 quantizes individual Wavelet transformcoefficients in conformance to quantization step widths set for theindividual sub-bands (FIG. 7 S108).

In the next step, the coding unit 102 divides each quantized Wavelettransform coefficient into blocks in a fixed size which are referred toas code blocks (FIG. 7 S109).

Then, the coding unit 102 expresses the quantized values of the Wavelettransform coefficients in the individual code blocks as a plurality ofbit planes, executes entropy coding in units of individual bit planesand thus generates code data in correspondence to the individual codeblocks (FIG. 7 S110).

Namely, coded data are generated in units of the individual code blocksfor all the sub-bands in each color component.

Next, the coding unit 102 performs arithmetic coding on the coded datathat have been generated as described above by employing an MQ coder(FIG. 7 S111).

The coding unit 102 then divides the coded data corresponding to all thecoded blocks in conformance to a predetermined layer structure (FIG. 8S112).

Next, the coding unit 102 groups the divided coded data into packetseach containing coded data with a common breakdown level, a common layerand a common color component (FIG. 8 S113). It is to be noted that thepackets are each constituted of a header and a body, with the attributeof the coded data constituting the body and the like stored in theheader.

In the following step, the coding unit 102 arranges the packets inconformance to a predetermined scalability (e.g., SNR scalability) andgenerates a code stream achieving a data volume corresponding to thedesired compression rate (FIG. 8 S114).

Then, the code stream thus generated is transmitted as the final codeddata to a client via the network interface unit 103 (FIG. 8 S115).

As explained above, the image data server 100 in the related art thatadopts the coding technology defined in the JPEG 2000, Part 1 realizescompression coding implemented at the desired compression rate requestedby a client by executing the processing in step S108 and subsequentsteps after sub-sampling the color difference components in thesub-sampling process executed in step S106 in FIG. 7.

However, when coding image data through compression coding as describedabove, the processing from FIG. 7 S102˜FIG. 8 S115 must be executedrepeatedly each time an image transmission request from a client isreceived. For this reason, a heavy load is placed on the coding unit 102and thus, a problem arises in that the image data server cannot respondquickly to a client's request for a transmission.

In addition, since it is necessary to record in advance uncoded,uncompressed image data in the image data server 100, the imagerecording device 101 must have a large storage capacity.

“The medium quality compression image data server” and the “high qualitycompression image data server” explained earlier that record coded dataobtained through compression coding do not require a recording devicewith a large capacity.

However, the “medium quality compression image data server” is onlycapable of transmitting coded data obtained by compression coding at apredetermined compression rate (approximately 1/10˜ 1/20). For thisreason, there are problems in that the data volume cannot be reducedwhen it is desired to transmit the image data with high efficiency andin that the image data server cannot respond to a client's request fortransmission of a high quality image or a high resolution image.

In addition, if a client requests a transmission of a low quality imageor a low resolution image, the “high quality compression image dataserver” must first decompress coded data having been performed withcompression coding and then re-code the data through the processing suchas that shown in FIG. 7 S102˜FIG. 8 S115. Thus, the image data servercannot respond quickly to the transmission request from a client. Thereis another problem in that since such re-coding cannot be implementedwithout first decompressing the data, the quality of the imagereproduced based upon the coded data resulting from the re-codingprocessing becomes poor compared to the quality of the image reproducedbased upon coded data obtained by compression coding of uncompressedimage data.

SUMMARY OF INVENTION

The present invention provides an image data coding apparatus capable ofcoding image data in conformance to a desired compression rate or adesired quality for the restored image with ease. It also provides animage data server capable of coding image data in conformance to thedesired compression rate or a desired quality for the restored imagewith ease and speed while minimizing the deterioration of the imagequality.

The image data coding apparatus according to the present inventionhaving a coding unit that generates coded data after generating atransform (or conversion) coefficient by implementing Wavelet transformon image data comprises a storage unit that stores coded data which thecoding unit generates by coding image data which have not beensub-sampled, an information-obtaining unit that obtainscompression-related information indicating a compression rate or arestored image quality and a re-coding unit that changes a sub-samplingratio of individual color components constituting the coded data storedin the storage unit in conformance to the compression-relatedinformation and generates new coded data.

In this image data coding apparatus, the information-obtaining unit mayobtain either compression-related information set by the operator orcompression-related information provided from an external apparatus. Inaddition, the new coded data generated by the re-coding unit may berecorded into an internal recording device or they may be transferred toan external apparatus.

In other words, the invention described above may be adopted in anelectronic camera having an operating unit operated to setcompression-related information or an image transfer apparatus (whichmay include an electronic camera having a coded data transfer function)having a function of receiving compression-related information providedby the transfer recipient.

As long as coded data can be recorded into a detachable recording medium(e.g., a PC card) even if the electronic camera does not have a codeddata transfer function, the coded data recorded in the recording mediumcan be transferred via a personal computer or the like. The operator ofsuch an electronic camera is able to set information indicating aspecific compression rate or a specific restored image quality(equivalent to the compression-related information) through a specificoperating unit and is able to output the desired coded data to therecording medium. Thus, coded data having been compressed in response toa request issued by the transfer recipient can be transferred byadopting the present invention described above in an electronic camerathat does not have the coded data transfer function, as well.

In the image data coding apparatus described above, the image datacomprise a brightness (or luminance) component and color differencecomponents, and the re-coding unit may determine a new sub-samplingratio to be set in conformance to characteristics of the image dataobtained based upon at least either the brightness component or thecolor difference components in addition to the compression-relatedinformation.

In other words, in this image data coding apparatus, the sub-samplingratio of the various color components constituting the coded data storedin the storage unit is changed in conformance to the characteristics ofthe image data as well as the compression-related information togenerate new coded data.

It is to be noted that the characteristics of the image data obtainedbased upon at least either the brightness component and the colordifference components include, for instance, the “rate of the volume ofthe color difference component information relative to the volume of theentire image data”.

In addition, the sub-sampling ratio should be determined incorrespondence to the “rate of the volume of the color differencecomponent information relative to the volume of the entire image data”so as to lower the ratio of the color difference components if the “rateof the volume of the color difference component information to thevolume of the entire image data” is low and so as to raise the ratio ofthe color difference component if the “rate of the volume of the colordifference component information to the volume of the entire image data”is high.

Furthermore, the coding unit may generate the coded data in units ofindividual tile images obtained by dividing the image data of the imagespace and the re-coding unit may change the sub-sampling ratio of thecolor components constituting coded data corresponding to each of thetile images in conformance to the characteristics of the tile images andthe compression-related information and generate new coded dataaccordingly.

In the image data coding apparatus described above, the re-coding unitmay convert data corresponding to a high frequency component in thecoded data stored in the storage unit to invalid data or it may deletethe data corresponding to the high-frequency component to generate newcoded data, and in such a case, the re-coding unit may change thesub-sampling ratio by increasing/decreasing the data to be converted toinvalid data or to be deleted.

The invalid data in this context may be, for instance, data continuouslyindicating 0.

The image data server according to the present invention having a codingunit that generates coded data after generating a transform coefficientby implementing Wavelet transform on image data comprises a storage unitthat stores coded data which the coding unit generates by coding imagedata which have not been sub-sampled, an information obtaining unit thatobtains an image transmission request issued by a client andcompression-related information indicating a compression rate or arestored image quality, a re-coding unit that changes a sub-samplingratio of individual color components constituting the coded data storedin the storage unit in conformance to the compression-relatedinformation and generates new coded data, and a transmission unit thattransmits the new coded data generated by the re-coding unit to aclient.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a functional block diagram of an image data server achieved inan embodiment of the present invention;

FIG. 2 presents a flowchart of the operation performed in the codingunit;

FIG. 3 is provided to facilitate an explanation of the processingthrough Wavelet transform executed by the coding unit;

FIG. 4 is provided to facilitate an explanation of another example ofthe processing through the Wavelet transform executed by the codingunit;

FIG. 5 presents a flowchart of the operation performed by the image dataserver when a client has issued a transmission request;

FIG. 6 presents examples of how the sub-sampling ratio may be set;

FIG. 7 presents a flowchart of the operation performed by an image dataserver in the related art which adopts the coding technology defined inthe JPEG 2000 Part 1;

FIG. 8 presents the flowchart (continued) of the operation performed bythe image data server in the related art which adopts the codingtechnology defined in the JPEG 2000 Part 1;

FIG. 9 is a functional block diagram of an image data server in therelated art;

FIG. 10 is provided to facilitate an explanation of the processingthrough Wavelet transform executed by the coding unit; and

FIG. 11 is provided to facilitate an explanation of sub-sampling ratios.

DESCRIPTION OF THE PREFERRED EMBODIMENT

The following is a detailed explanation of an embodiment of the presentinvention, given in reference to the drawings.

FIG. 1 is a functional block diagram of the image data server achievedin the embodiment of the present invention.

An image data server 10 in FIG. 1 comprises a coding unit 11, a storageunit 12, a re-coding unit 13 and a network interface unit 14.

It is to be noted that the image data server 10 may be an image dataserver having a function of generating coded data, a function of storingthe coded data, a function of obtaining compression-related informationand a function of generating new coded data, or it may be an image dataserver having an additional function of transmitting the new coded datato a client. The network interface unit 14 is connected to any ofvarious types of networks such as the Internet and an intra-net, andvarious types of information are exchanged via the network interfaceunit 14. The network interface unit 14 embodies a lower-order concept ofthe information obtaining unit.

The coding unit 11 in the image data server 10 achieved in theembodiment codes image data without sub-sampling them, and adopts thecoding technology defined in the JPEG 2000 Part 1 as does the codingunit 102 in the image data server 100 in the related art.

FIG. 2 presents a flowchart of the operation performed by the codingunit 11.

It is to be noted that the same step numbers as those in FIGS. 7 and 8are assigned to the steps in FIG. 2 in which processing similar to thatperformed by the coding unit 102 in the image data server 100 in therelated art is implemented.

The following is an explanation of the operation performed by the codingunit 11, given in reference to FIG. 2.

First, the coding unit 11 divides the image data into a plurality oftile images (FIG. 2 S103) and generates image data in the YCbCrcolorimetric system by implementing color coordinate conversion on theindividual tile images (FIG. 2 S104) as does the coding unit 102.However, it does not sub-sample the image data by culling the colordifference components at this time. (B) of FIG. 3 shows image data inthe YCbCr colorimetric system generated through the color coordinateconversion.

As in the image data server 100 in the related art, the sub-samplingratio of the color components (the Y component, the Cb component and theCr component) of the image data constituting each tile image is setafter the compression-related information is provided by a client in theimage data server 10 in the embodiment. While a uniform sub-samplingratio may be set for all the tile images in conformance to thecompression-related information alone, the sub-sampling ratio is set foreach tile image in conformance to the characteristics of the tile imageas well as the compression-related information in the embodiment.

In this embodiment, the rate of the volume of the color differencecomponent information relative to the volume of the image data in theYCbCr colorimetric system is used as the characteristics of each tileimage and this rate is indicated as the value of an index (hereafterreferred to as an “image characteristics index”) calculated by usingstandard deviation σ (Y), σ(Cb) and σ(Cr) manifesting at the Y plane,the Cb plane and the Cr plane respectively. It is to be noted that suchan image characteristics index should be preferably calculated by usingthe image data in the YCbCr colorimetric system equivalent to that tileimage, which has not been coded yet.

Accordingly, the image characteristics indices for the individual tileimages are calculated in the embodiment when the image data in the YCbCrcolorimetric system for the individual tile images are generated.

Namely, upon generating the image data in the YCbCr colorimetric systemin correspondence to each tile image as described above, the coding unit11 calculates the image characteristic indices for the individual tileimages (FIG. 2 S1).

For instance, an image characteristics index obtained through such aprocess may be any one of image characteristics indices S1˜S3respectively calculated through the following expressions 1˜3.Image characteristics index S1:σ(Y)/(σ(Y)+σ(Cb)+σ(Cr))  expression 1Image characteristics index S2:σ²(Y)/(σ²(Y)+σ²(Cb)+σ²(Cr))  expression 2Image characteristics index S3:σ(Y)/(σ(Cb)+σ(Cr))  expression 3

In addition, with Nx representing the number of pixels along the x axisin the image data, Ny representing the number of pixels along the y axisand Y[i, j], Cb[i, j] and Cr [i, j] respectively representing the valuesof the Y component, the Cb component and the Cr component respectivelyat a given pixel, the averages <Y>, <Cb> and <Cr> on the Y plane, the Cbplane and the Cr plane respectively and the standard deviations σ(Y),σ(Cb) and σ(Cr) on the individual planes can be calculated through thefollowing expressions 4˜9. $\begin{matrix}{{\sigma(Y)} = \sqrt{\frac{1}{NxNy}{\sum\limits_{i = 1}^{Nx}{\sum\limits_{j = 1}^{Ny}\left( {{Y\left\lbrack {i,j} \right\rbrack} - \left\langle Y \right\rangle} \right)^{2}}}}} & {{expression}\quad 4} \\{{\sigma({Cb})} = \sqrt{\frac{1}{NxNy}{\sum\limits_{i = 1}^{Nx}{\sum\limits_{j = 1}^{Ny}\left( {{{Cb}\left\lbrack {i,j} \right\rbrack} - \left\langle {Cb} \right\rangle} \right)^{2}}}}} & {{expression}\quad 5} \\{{\sigma({Cr})} = \sqrt{\frac{1}{NxNy}{\sum\limits_{i = 1}^{Nx}{\sum\limits_{j = 1}^{Ny}\left( {{{Cr}\left\lbrack {i,j} \right\rbrack} - \left\langle {Cr} \right\rangle} \right)^{2}}}}} & {{expression}\quad 6} \\{\left\langle Y \right\rangle = {\frac{1}{NxNy}{\sum\limits_{i = 1}^{Nx}{\sum\limits_{j = 1}^{Ny}{Y\left\lbrack {i,j} \right\rbrack}}}}} & {{expression}\quad 7} \\{\left\langle {Cb} \right\rangle = {\frac{1}{NxNy}{\sum\limits_{i = 1}^{Nx}{\sum\limits_{j = 1}^{Ny}{{Cb}\left\lbrack {i,j} \right\rbrack}}}}} & {{expression}\quad 8} \\{\left\langle {Cr} \right\rangle = {\frac{1}{NxNy}{\sum\limits_{i = 1}^{Nx}{\sum\limits_{j = 1}^{Ny}{{Cr}\left\lbrack {i,j} \right\rbrack}}}}} & {{expression}\quad 9}\end{matrix}$

Next, as does the coding unit 102, the coding unit 11 breaks down eachof the color components constituting the image data in the YCbCrcolorimetric system corresponding to each tile image into a plurality ofsub-bands through discrete Wavelet transform implemented along twodirections, i.e., along the vertical direction and the horizontaldirection (FIG. 2 S107). (C) of FIG. 3 shows each color component in theimage data broken down into a plurality of sub-bands through thediscrete Wavelet transform implemented along the vertical direction andthe horizontal direction.

It is to be noted that while the individual color components (the Ycomponent, the Cb component and the Cr component) corresponding to eachtile image are broken down through the discrete Wavelet transformimplemented along the vertical direction and the horizontal direction inthis example, the discrete Wavelet transform may be implemented on theindividual color components corresponding to each tile image along asingle direction at a time, first along the horizontal direction andthen along the vertical direction, as shown in (C-1) and (C-2) in FIG.4, instead.

Next, as does the coding unit 102, the coding unit 11 quantizes Wavelettransform coefficients (FIG. 2 S108) and divides the quantized Wavelettransform coefficients into code blocks (FIG. 2 S109).

Then, as does the coding unit 102, the coding unit 11 expresses thequantized values of the Wavelet transform coefficients in the individualcoded blocks as a plurality of bit planes, implements entropy coding inunits of individual bit planes and generates coded data for each codedblock (FIG. 2 S110).

In the next step, as does the coding unit 102, the coding unit 11implements mathematical coding on the coded data by employing an MQcoder (FIG. 2 S111).

Next, as does the coding unit 102, the coding unit 11 divides the codeddata in conformance to a specific layer structure (FIG. 2 S112), andthen groups the divided coded data into packets each containing codeddata sharing a common breakdown level, a common layer and a common colorcomponent (FIG. 2 S113).

Lastly, the coding unit 11 stores the packets corresponding to theindividual tile images and the values of the image characteristicsindices calculated in FIG. 2 S1 into the storage unit 12 (FIG. 2 S2).

Namely, the coded data in the packet format corresponding to theindividual tile images and the image characteristics index values arestored into the storage unit 12 in advance before a transmission requestfrom a client is received.

FIG. 5 presents a flowchart of the operation performed by the image dataserver 10 when there is a transmission request from a client.

The following is an explanation of the operation performed by the imagedata server 10 when a client requests a transmission, given in referenceto FIG. 5.

First, the image data server 10 makes a decision via the networkinterface unit 14 as to whether or not a request for an imagetransmission has been issued by a client (FIG. 5 S10).

If it is decided that an image transmission request has been issued, theimage data server 10 obtains the compression-related informationprovided by a client (FIG. 5 S11).

It is to be noted that while the compression-related information mayindicate a compression rate level or a level of the quality of therestored image as in the image data server 100 in the related art, thecompression-related information provided in this example indicates adesired compression rate.

However, in order to simplify the explanation, it is assumed in thisexample that a plurality of compression rates are made available inadvance, one of these compression rates is selected by a client and theselected compression rate is provided as the compression-relatedinformation to the image data server 10. For instance, three differentcompression rates 4 bpp, 2 bpp and 1 bpp may be made available inadvance or four compression rates 4 bpp (bits/pixel), 2 bpp, 1 bpp and0.5 bpp may be made available in advance.

Once such compression-related information is obtained, the re-codingunit 13 reads out the packets and the image characteristics index valuesin correspondence to the individual tile images making up the image tobe transmitted from the storage unit 12. Then, the re-coding unit 13sets the sub-sampling ratio of the color components (the Y component,the Cb component and the Cr component) in conformance to the imagecharacteristics index value and the compression related information foreach tile image (FIG. 5 S12).

For instance, if the image characteristics index value is equal to orlarger than a predetermined value, the re-coding unit 13 judges that therate of the volume of the color difference component information is lowand accordingly sets the sub-sampling ratio so as to lower the ratio ofthe volume of the color difference component information. If, on theother hand, the image characteristics index value is smaller than thepredetermined value, it judges that the rate of the volume of the colordifference component information is high, and accordingly sets thesub-sampling ratio to raise the ratio of the volume of the colordifference component information.

FIG. 6 presents examples of sub-sampling ratio settings.

It is to be noted that while FIG. 6 shows examples in which thesub-sampling ratio is set to either “4:4:4” or “4:2:0”, “4:2:2”representing an intermediate ratio may also be used to allow one of thethree sub-sampling ratios to be selected.

Next, the re-coding unit 13 generates a code stream conforming to apredetermined level of scalability by replacing the packetscorresponding to a low breakdown level (the highfrequency component)among the packets containing the color difference component data in eachtile image with packets each containing body data that indicate thevalue of 0 only (hereafter referred to as “all 0 packets”) (FIG. 5 S13).

It is to be noted that the sub-sampling ratio setting determines exactlywhich low-breakdown level packets are to be replaced with “all 0packets”.

For instance, if the color difference components of each tile image arebroken down into breakdown level 0˜breakdown level 3 as shown in (C) ofFIG. 3, the re-coding unit 13 replaces the packets corresponding tobreakdown level 3 (the lowest breakdown level) of the Cb component andthe Cr component in each tile image for which the sub-sampling ratios ofY, Cb and Cr is set to “4:2:0” with “all 0 packets”.

In addition, when a given tile image has its sub-sampling ratio for Y,Cb and Cr set to “4:2:2”, the packets corresponding to HH3 and HL3 atbreakdown level 3 (the lowest breakdown level) of the Cb component andthe Cr component are replaced with “all 0 packets”.

It is to be noted that none of the packets in a tile image having asub-sampling ratio for Y, Cb and Cr set to “4:4:4” are replaced with“all 0 packets” by the re-coding unit 13.

The code streams generated as explained above are transmitted to aclient via the network interface unit 14. During the transmissionprocess, the data transmission is terminated once the volume of thetransmitted data in the code stream reaches of volume corresponding tothe desired compression rate (FIG. 5 S14).

When the data in the code streams that have been transmitted asdescribed above are decoded on a client side, the “all 0 packets” arehandled as invalid data that do not affect the quality of the imageobtained through the decoding process.

Accordingly, the quality of the image restored by using the code streamstransmitted by the image data server 10 in the embodiment can beconsidered comparable to the quality of the image restored with the codestream data obtained by sub-sampling the color difference components inthe image data server 100 in the related art.

In addition, since the volume of the data corresponding to the “all 0packets” is insignificant, the volume of the code stream datatransmitted by the image data server 10 in the embodiment can beconsidered approximately equal to the volume of the code stream dataobtained by sub-sampling the color difference components in the imagedata server 100 in the related art.

In other words, the image data server 10 in the embodiment replaces thepackets of color different data corresponding to a low breakdown levelwith “all 0 packets” in conformance to the sub-sampling ratio settinginstead of sub-sampling the color difference components in conformanceto the sub-sampling ratio setting, to achieve similar compressionresults.

Furthermore, the processing for replacing the color differencecomponents packets corresponding to the low breakdown level with “all 0packets” in conformance to the sub-sampling ratio setting is executedwithout decoding the coded data stored in the storage unit 12 in advanceas packets. Thus, since it is not necessary to repeatedly generatepackets each time a transmission request from a client is received oncethe packets are generated and stored in the storage unit 12, the imagedata server 10 in the embodiment is capable of responding to a client'srequest for a transmission more speedily than the image data server 100in the related art.

In addition, since packet format image data are stored in the storageunit 12 at the image data server 10 in the embodiment, the storage unit12 does not need to have a recording capacity as large as that requiredin an image data server in which uncoded, uncompressed image data arerecorded.

Moreover, since the sub-sampling ratio is set for each tile image basedupon the characteristics of the image, and more specifically, thecharacteristics of the particular tile image as well as thecompression-related information in the image data server 10 in theembodiment, the volumes of information corresponding to the individualcolor components can be adjusted to achieve correct proportions for eachtile image. As a result, the quality of the image having undergone thecoding process does not deteriorate as much as when image data are codedat a uniform sub-sampling ratio set for all the tile images.

It is to be noted that while a code stream is generated in theembodiment by replacing the color difference components packetscorresponding to the low breakdown level with “all 0 packets”, a codestream may be generated by eliminating the color difference componentspackets corresponding to the low breakdown level. However, when thecolor difference components packets corresponding to the low breakdownlevel are eliminated in this manner, information indicating suchdeletion must be added into the header of the code stream.

In addition, while packets corresponding to the individual tile imagesare stored in the storage unit 12 and a code stream is generated by there-coding unit 13 in the embodiment, the code stream generation by there-coding unit 13 may be omitted if a single scalability mode isselected in advance by executing processing for rearranging the packetsin conformance to the scalability mode to generate a code stream at thecoding unit 11 in advance and by storing the code stream thus generatedin the storage unit 12.

Moreover, while image data in the YCbCr colorimetric system aregenerated through the color coordinate conversion implemented on tileimages in the embodiment, image data may instead be generated in any ofvarious colorimetric systems including the YUV colorimetric system, theLab colorimetric system, the GCbCr colorimetric system, the YIQcolorimetric system and the G(R-G) (B-G) colorimetric system.

While a single piece of compression-related information is obtained forthe entire image data to be transmitted and the commoncompression-related information is utilized when setting thesub-sampling ratios for the individual tile images in the embodiment,compression-related information may instead be individually obtained foreach tile image to be used when setting the sub-sampling ratio of thetile image.

The coding function achieved by an image data coding apparatus isadopted in an image data server in the embodiment. However, this imagedata coding apparatus may be adopted in any system other than an imagedata server as long as the system codes image data through Wavelettransform (in conformance to JPEG 2000 Part 1, for instance). Suchsystems include an electronic camera provided with an operating unitoperated to set compression-related information and an image datatransfer apparatus (which may include an electronic camera having acoded data transfer function) that achieves a function of receivingcompression-related information provided by a transfer recipient.

As explained above, coded data corresponding to the compression rateindicated by compression-related information or coded data correspondingto the quality of the restored image indicated by thecompression-related information can be generated as new coded datasimply by changing the sub-sampling ratio of the various colorcomponents constituting pre-recorded coded data in conformance to thecompression-related information in the embodiment. In other words, codeddata corresponding to the desired compression rate or coded datacorresponding to the desired quality for the restored image can begenerated with ease and speed. As a result, the length of the processingtime elapsing between the reception of a request for an imagetransmission issued by a client and the actual transmission can bereduced.

Furthermore, since the sub-sampling ratio of the various colorcomponents constituting the coded data can be varied in conformance tothe characteristics of the image data in addition to thecompression-related information, the volumes of informationcorresponding to the individual color components can be adjusted toachieve correct proportions, which makes it possible to generate codeddata that will minimize the deterioration of the image quality. Inparticular, since the sub-sampling ratio of the color componentsconstituting the coded data in each tile image can be adjusted inconformance to the characteristics of the tile image and thecompression-related information, the distribution of the informationvolumes in small areas of the image can be adjusted, and thus, codeddata that will minimize the deterioration of the image quality can begenerated.

Moreover, an operation that achieves an effect similar to that realizedby changing the sub-sampling ratio of the individual color componentsconstituting coded data can be executed with ease and with speed. Inaddition, since coded data generated by the coding unit can be stored,the recording device does not need to have a large capacity which wouldotherwise be required to record uncoded, uncompressed image data.

The above described embodiment is an example, and various modificationscan be made without departing from the spirit and scope of theinvention.

1. An image data coding apparatus having a coding unit that generatescoded data after generating a transform coefficient by implementingWavelet transform on image data, comprising: a storage unit that storescoded data which the coding unit generates by coding image data whichhave not been sub-sampled; an information obtaining unit that obtainscompression-related information indicating a compression rate or arestored image quality; and a re-coding unit that changes a sub-samplingratio of individual color components constituting the coded data storedin the storage unit in conformance to the compression-relatedinformation and generates new coded data.
 2. An image data codingapparatus according to claim 1, wherein: the image data comprise abrightness component and a color difference components; and there-coding unit determines a new sub-sampling ratio to be set inconformance to characteristics of the image data obtained based upon atleast either the brightness component or the color difference componentsin addition to the compression-related information.
 3. An image datacoding apparatus according to claim 2, wherein: the coding unitgenerates coded data in units of individual tile images obtained bydividing the image data on an image space; and the re-coding unitchanges the sub-sampling ratio of the color components constitutingcoded data corresponding to each of the tile images in conformance tocharacteristics of the tile image and the compression-relatedinformation and generates new coded data accordingly.
 4. An image datacoding apparatus according to claim 1, wherein: the re-coding unitconverts data corresponding to a high frequency component in the codeddata stored in the storage unit to invalid data or eliminates the datacorresponding to the high-frequency component to generate new codeddata, and change the sub-sampling ratio by increasing/decreasing thedata to be converted to invalid data or to be deleted.
 5. An image datacoding apparatus according to claim 2, wherein: the re-coding unitconverts data corresponding to a high frequency component in the codeddata stored in the storage unit to invalid data or eliminates the datacorresponding to the high-frequency component to generate new codeddata, and change the sub-sampling ratio by increasing/decreasing thedata to be converted to invalid data or to be deleted.
 6. An image datacoding apparatus according to claim 3, wherein: the re-coding unitconverts data corresponding to a high frequency component in the codeddata stored in the storage unit to invalid data or eliminates the datacorresponding to the high-frequency component to generate new codeddata, and change the sub-sampling ratio by increasing/decreasing thedata to be converted to invalid data or to be deleted.
 7. An image dataserver having a coding unit that generates coded data after generating atransform coefficient by implementing Wavelet transform on image data,comprising: a storage unit that stores coded data which the coding unitgenerates by coding image data which have not been sub-sampled; aninformation obtaining unit that obtains an image transmission requestissued by a client and compression-related information indicating acompression rate or a restored image quality; a re-coding unit thatchanges a sub-sampling ratio of individual color components constitutingthe coded data stored in the storage unit in conformance to thecompression-related information and generates new coded data; and atransmission unit that transmits the new coded data generated by there-coding unit to a client.